Mechanistic study of mixed lithium halides solid state electrolytes

TL;DR

This study uses machine learning potentials to explore how alloying and structural changes in lithium halide solid electrolytes affect lithium conductivity, providing insights for designing better materials.

Contribution

It applies a universal MLIP to analyze the structure-property relations in Li-Y-X halide alloys, revealing how composition influences lattice and conductivity.

Findings

Alloying modulates lattice parameters and can induce symmetry transitions.

Chemical composition and lattice effects on conductivity tend to offset each other.

Y-In substitution slightly increases conductivity in certain phases.

Abstract

Lithium halides with the general formula Li$_x$M$_y$X$_6$, where M indicates transition metal ions and X halide anions are very actively studied as solid-state electrolytes, because of relatively low cost, high stability and Li conductivity. The structure and properties of these halide-based solid electrolytes (HSE) can be tuned by alloying, e.g. using different halides and/or transition metals simultaneously. The large chemical space is difficult to sample by experiments, making simulations based on broadly applicable machine-learning interatomic potentials (MLIPs) a promising approach to elucidate structure-property relations, and facilitate the design of better-performing compositions. Here we focus on the Li$_3$YCl$_{6x}$Br$_{6(1-x)}$ system, for which reliable experimental data exists, and use the recently-developed PET-MAD universal MLIP to investigate the structure of the alloy, the interplay of crystalline lattice, volume and chemical composition, and its effect on Li conductivity. We find that the distribution of Cl and Br atoms is only weakly correlated, and that the primary effect of alloying is to modulate the lattice parameter -- although it can also trigger transition between different lattice symmetries. By comparing constant-volume and constant-pressure simulations, we disentangle the effect of lattice parameter and chemical composition on the conductivity, finding that the two effects compensate each other, reducing the overall dependency of conductivity on alloy composition. We also study the effect of Y-In metal substitution finding a small increase in the conductivity for the C2/m phase at 25\% In content, and an overall higher conductivity for the P$\bar{3}$m1 phase.